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Greenhouse gas emissions from soils increased by earthworms: opening a can of worms

12 Feb 2013, 10:15

Ingrid Lubbers and colleagues

The results of a new study on the effect of earthworms on
greenhouse gas emissions attracted a fair bit of attention last
week. Here, the researchers respond to a few of the most popular
comments and misconceptions about their findings.

Last week, our
study on the effect of earthworms on soil greenhouse emissions
appeared in Nature Climate Change. Although we were pleased about
all the attention focused on our work in
Carbon Brief and the
Guardian, almost inevitably some misunderstandings showed up in
online discussions. Here are our answers to six of the most
frequently posted comments and questions:

1: Earthworms cause global warming, not humans, so we
can all stop worrying now

This comment, which also comes in another form - that we're
making excuses for human-caused global warming - is without a doubt
the most serious misunderstanding of our work.

Humans cause global warming, not earthworms. Since earthworms
have been around for hundreds of millions of years, it would be
naïve to assume they suddenly started to cause global change
somewhere in the previous century.

In the case of carbon dioxide, the largest cause of increasing
atmospheric concentrations is burning fossil fuels and land-use
change. Whereas with nitrous oxide, the largest cause is the
enormous amount of nitrogen fertilizer that mankind applies to
agricultural soils.

Our study shows earthworms are an important actor through which
humans can cause nitrous oxide emissions. It also suggests that,
due to the increasing habitat for earthworms over the coming
decades, earthworm-induced emissions may increase. But these
emissions are largely conditional on humans adding fertilizer to
agricultural systems.

2: The authors ignore all the positive things earthworms
do in the soil

We certainly do not - not in our Nature Climate Change article,
and not in the many studies on earthworms that we published in the
past. Agriculture would be much worse off without earthworms. They
have many positive effects on soil fertility, drainage, and on
overall soil quality.

In our article we just show that we cannot give them credit for
decreasing greenhouse gas emissions. Rather, they appear to
increase greenhouse gas emissions as an unfortunate side-effect of
their positive effects on soil fertility.

3: How much do earthworms contribute to greenhouse gas
emissions, compared to fossil fuels?

This is an understandable question - but unfortunately the answer
is very complex. It is estimated that carbon dioxide from fossil
fuel burning causes around 60 to 70 per cent of global warming, and
soil nitrous oxide emissions cause around five to seven per cent.
It is difficult to say what part of that five to seven per cent is
due to earthworms, as detailed data on global earthworm populations
is lacking.

But a rough guess based on our findings is that
earthworm-induced emissions cause around one per cent of global
warming. This may not seem much at first glance, but these
emissions are only marginally less than all
fossil fuel used in global air travel.

The team of scientists found earthworms increase carbon
dioxide emissions from soil by 33 per cent and nitrous oxide
emissions by 42 per cent.

4: The study shouldn't have been published, as the
results could be misused by special interest groups

The results of scientific studies can be selectively
misinterpreted. But we believe that should not keep anyone from
publishing good and important science, provided they are careful in
explaining its implications and limitations.

We strongly feel that, if we would choose to not publish our
work because it might be misused, science would have lost.

5: You are not helping the cause of organic
agriculture

The purpose of our study was to determine the role of earthworms
in the soil greenhouse gas balance, not to endorse any particular
form of agriculture. We don't agree that our results reflect badly
on organic agriculture.

It is true that organically farmed soils generally contain more
earthworms than conventionally managed soils. It is also true that
our results therefore suggest that earthworms will have a
relatively larger impact on nitrous oxide emissions from organic
agriculture.

That said, recent studies suggest that emissions of nitrous
oxide from organic systems might be lower than in conventional
farming systems and more carbon is stored in the topsoil of organic
systems, despite higher earthworm densities. This might partly be
because, as we have pointed out in our article, none of the studies
that we could use in our meta-analysis accounted for the fact that
- especially in organic systems - earthworms can increase crop
growth.

This might in turn lead to increased soil organic matter storage
and increased nitrogen uptake, and therefore in lower greenhouse
gas emissions. This needs to be studied further in the future.

6: Does this mean that we should now kill all
earthworms?

Certainly not! And we couldn't if we wanted to. Any farmer knows
that earthworms are beneficial for agricultural productivity and
soil quality. But the evidence suggests that we should not delude
ourselves into thinking that they can reduce greenhouse gas
emissions for us. We will have to do that ourselves.

Can of worms

We hope that this has shed some more light on the main outcomes
and limitations of our study, and that we succeeded in handling the
can of worms that we opened.